Martial Artists have the upper hand due to the fact that they are trained in numerous types of fighting.
For example, let's say that a fight broke out b/w a skilled boxer and a skilled martial artist. The boxer is 195lbs and stands at 6'2'' while the martial artist is 186lbs and stands at 5'10''. The martial artist has a black belt in jujutsu and he's extremely FAST. However, the boxer is a lot more conditioned that the martial artist.
The thing that people have to keep in mind is that Boxers need space in order to fight because that's how they're trained while martial artists are trained in both open space and close space.
The martial artist will win because he has the upper hand.